Basic Player Requirements
To be eligible for the UEFA Champions League, a player must be at least 16 years old, hold a valid passport from their country of nationality, and be a professional or semi-professional footballer. Players must be under contract with a club that has qualified for the competition. The player's club must also be a member of UEFA and registered with their national football association.
Registration and Documentation
Players must be officially registered with both their club and their national football association before the competition begins. Clubs must submit their squad lists to UEFA by specific deadlines set before each season. All player information, including passport details and registration documents, must be accurate and verified. Players cannot be registered for more than one club in the same season of the Champions League.
Squad List Rules
Each club can register up to 25 players on their official Champions League squad list. This list can include players from the club's academy system. Players can be added or removed from the squad list at certain points during the season, but changes must follow UEFA regulations. If a player is not on the registered squad list, they cannot play in any Champions League match.
Loan Players and Transfers
A player who was loaned to another club earlier in the season cannot play against their parent club in the Champions League. Players who transfer between clubs during the season may be eligible to play for their new club if they are registered before the deadline. Previous participation in the competition with another club does not affect eligibility for a new club.
International Player Status
Players of any nationality can play in the Champions League as long as they meet the basic requirements. There are no restrictions based on where a player was born or their nationality. A player represents the club they are registered with, not their country, in the Champions League. 
Answer